A Horn Antenna for 2.4 GHz.
This Antenna is not really practical for AO-40
reception, but horn antennas have a number of qualities useful in microwave
antenna testing and noise figure measurements.
This antenna was designed for use on an antenna test range set up for AMSAT UK
at their annual colloquium held at Surrey University in July 2001.
The dimensions for the horn and the waveguide were generated from software
downloaded from the W1GHZ web site.
Because the gain of a horn antenna can be accurately calculated from its dimensions, it can be used as a standard gain antenna. Antennas under test can be directly compared with the horn and their gain and polarity can measured relative to the performance of the standard horn.
The Horn below has a gain of 16dBi
The waveguide has a cut-off frequency of 2.0 GHz
The waveguide has been constructed from a hard brass alloy. The dimensions were chosen to provide a waveguide cut-off at 2GHz. When making antenna measurements in a built up area I thought this may offer some rejection of Cell phone signals around 1800MHz.
Another application of a horn antenna is the
measurement of system noise figure by using a comparison of cold sky and 'hot'
ground.
I have tried this before with a dish antenna, only to discover that the
overspill from the dish feed caused the cold sky measurement to significantly
affected by noise from the ground. The solution is to use an antenna which have
a very unidirectional pattern. A horn antenna meets this requirement.
The transistion from coax to waveguide uses an SMA connector with a length of 4mm brass rod soldered onto the SMA. A set of 3 x 25mm long M4 matching screws spaced at 0.125 guide wavelengths along the waveguide give a 20dB return loss at 2.400 GHz.
The waveguide internal dimension is 76 x 38mm.